There are other bands of note that were not included in the above list, but deserve recognition for their very strong progressive influence in the music scene. I add these:

Rush - namely the album 2112, but any album is sufficient, and is definitely the very definition of progressive rock music. Songs like Tom Sawyer are a prime example of the sound Rush strove for.

Radiohead - at last, good prog rock hits my time! As a twenty year old, Rush was a little before my time, even though I can still remember hearing songs like Closer To The Heart. But Radiohead's earlier albums weren't too much into the prog rock feel, more of an alternative bent on the early nineties rock out there. But with the albums after that (think the recently released Kid A and OK Computer) Radiohead greatly diversifies. Even Geddy Lee (lead singer and bass in Rush) loves those albums, and says that Radiohead carries on in the finest prog rock tradition.